* Add Spec to some user-facing streams

Where the generated code replaces Request with a stream, expose the
Spec.

* Expose peer information to servers and clients

On requests and streams, expose the peer's address. For clients, the
address is the host or host:port, parsed from the URL. For servers, the
address is an IP:port pair, provided by `net/http` as
`Request.RemoteAddr`.

In #357, a handful of users asked for this information for logging,
per-IP rate limiting, and a variety of other server-side concerns. It's
also necessary for OpenTelemetry interceptors (#344).

Fixes #357.